It is well known in the field of reinforced concrete beams that the use of longitudinally extending steel strands in the lower portion of the beam will increase the strength of the beam; and that the strength will be furher increased by depressing or "harping" the steel strands toward the middle of the beam. This is described in many texts on concrete, including, for example, Modern Prestressed Concrete, by James R. Libby, Van Nostrand Reinhold Co., N.Y., N.Y., 1984. The harping of the steel strands in the reinforced concrete is very advantageous, as it increases the strength of the beams significantly. Thus, for example, a 60-foot beam having a cross section of thirty-one (31) inches by sixteen (16) inches, and using sixteen (16) conventional linear steel strands could be replaced by a beam having a width of only twelve (12) inches, and having only fourteen (14) steel strands, if the steel strands are appropriately harped, or depressed toward the bottom of the beam, at the center of the beam.
However, relatively high forces are involved in tensioning the steel strands which are used in such concrete beams, and the deflecting of the steel strands is not easily accomplished at the job site. The inventors are familiar with one effort to make harped beams on the job site using plastic tubes to hold the steel tensioning cables or strands at the proper elevations at the middle and at the beam ends in several aligned beam forms, and then attempting to tension the strands. This effort was unsuccessful, and no other efforts to make harped beams at the job site are known to the inventors.
Harped concrete beams have normally been made at fixed casting yards. At these fixed plant locations, heavy, permanent, steel bed are normally employed, and these beds are formed of welded steel plates with the beds being several hundred feet long so that a number of beams may be constructed in line using a single set of tensioned steel strands. Following tensioning, the strands are harped, or pulled down by small fixtures which engage one or two sets of vertically aligned strands. In some cases, rollers which are employed in harping several horizontally and vertically spaced cable strands are left in the beam, thereby increasing the cost of the beam. In view of the permanent nature of the installation, permanent utility installations are employed, and powerful permanent installations are used. In many cases the harping method involves arrangements located in permanent pits below the steel beds. In one recent case involving necessary relocation of such a fixed casting yard, the need to cut apart the huge permanent one-piece steel bed, and the necessary re-welding and grinding, means that the local relocation costs will exceed one million dollars.
It is also noted that, when harped concrete beams are purchased from a fixed casting yard, the high cost and difficulties of moving large numbers of big heavy beams which may be sixty feet or more in length, are encountered. These may be so substantial that construction companies often prefer to cast their own un-harped beams locally, using a portable bed, at the construction site. One such portable bed is shown in U.S. Pat. No. 4,149,306.
Accordingly, a principal object of the present invention is to provide a simple, portable system for producing long span, harped, precast concrete beams, which may be accomplished at the construction site.